The present invention relates to a data communications system employed in a CATV system or the like.
In the case of a chargeable program in a CATV system, a scrambling signal is superposed on a TV signal transmitted from the center in each channel for which a charge is imposed, and at each subscriber's terminal unit there is provided a descrambler with which, for authorized channels, reproducible picture signals are obtained.
However, this system is disadvantageous in the following points: Since the TV signal is broken before transmission from the center and is reconstituted at the subscriber's terminal unit, the picture (or sound) quality is unavoidably reduced. The terminal units have a high manufacturing cost due to the complexity of the descrambling function. Furthermore, the system requires complex data communications operations to control the descrambling function. Also, it is possible for a dishonest subscriber to provide his own descrambling unit and thus avoid the charges for pay channels.
In order to overcome these difficulties, a method has been proposed in which the system is simplified by providing a main program reception control system at a distribution unit for plural terminal units at a tap-off position of the main line. In this case, each terminal unit is furnished with a converter for converting frequency-multiplexed TV signals of multiple channels into viewable channel signals.
Even in this case, however, it is necessary to collect up-data from the terminal units. Employment of the conventional data communications system between the center and the plurality of terminal units for such a distribution unit is not practical unless the equipment is extensively modified.